I'm not sure if I have enough information for anyone to help; but here goes. We removed the drywall on the outside wall of the basement corner shower (pipes leaked but are now fixed). After we removed the drywall we were able to see under the corner shower. The base is about 4 inches high and we can see the drain pipe coming out of the floor. The problem is that there is a large hole in the concrete where the pipe is coming out. Looks like the installer tied into the sewer line but didn't fill the hole with concrete. We are just wondering if could be a problem. There has been no water coming up but we can't tell if it is damp down the hole. It is about 1' x 1' in size - do not know how deep. We live in an area with lots of radon and would like to patch this hole up. I think we would have to remove the shower to do so. Any thoughts? Should we just leave it? Could it possibly have a purpose? We want to put drywall back on the wall but don't want to do that until we figure this out.

Welcome to the site: When they prep the floor for concrete the rough plumbing is put in first and they block it out to leave the hole so the plumber can hook up the shower when the room is finished.
It used to be standard to leave it open like you have and in many places it now has to be filled in. A quick call to the city inspectors for suggestions may be your best bet. If you take the pan out and fill it, you would have to be sure the pipe stays put or you could have a bigger problem.

Hi Neal. Thank you for your response! Now we know why the hole is there. It sounds like moving the shower pan can be quite the process if we aren't careful. I'll take your suggestion and phone the City. As I said before we are worried about radon levels but at the same time if the hole is okay we wouldn't mind just leaving it. Thanks again!!

I have hit this situation in the past. Took out a pre-fab shower to install a fill tub with tiled walls. There was a big hole. Worse, the foot of the new tub had nothing to sit on. Knowing the hole was for plumbing access, we dug out and poured a sonotube (reinforced with re-bar) at the point where the tub foot had to rest. Also installed a ledger around the tub preimeter to add support.

Finished the installation and moved on. Wish I had pics because the hole was a lot bigger than 1'X1'